Fuel injectors having piezoelectric actuators are known and are used in various applications. A typical piezoelectric actuator arrangement is made of a stack of piezoelectric material wafers. When exposed to an electrical field, the piezoelectric material in the actuator undergoes a physical dimension change, which causes an overall extension of the actuator. The displacement caused by the extension of the actuator is used to actuate internal components of the system in which it is arranged, for example, a fuel injector during an injection event.
Although piezoelectric actuators can yield relatively high actuation force when activated, the magnitude of the actuator force decreases dramatically as the displacement or stroke of the actuator increases. For example, although a typical piezoelectric actuator may be capable of producing 2 kN of force at the beginning of its stroke, its force output may decrease during its stroke and be zero at a stroke of about 40 μm. Thus, fuel injectors having mechanical couplings to transfer the actuator displacement to other portions of the fuel injector may lack sufficient actuation stroke or lack sufficient force for larger strokes. As can be appreciated, although the high force over a small displacement may be sufficient for a particular fuel injector application, its general applicability depends on the arrangement of the components to be displaced under the force of the actuator. For example, in fuel injectors, which is a common application for such actuators, the force of the actuator over its stroke may be sufficient for relatively smaller fuel injectors, or fuel injectors operating at relatively low fuel pressures, such as those used in engines with smaller displacements. However, it may be unsuitable for applications requiring larger fuel injectors or fuel injectors operating at relatively high fuel pressures.
Because of the need to increase the force of piezoelectric actuators over longer strokes, or alternatively the need to increase the forceful stroke of such actuators, known applications have used hydraulic amplification arrangements. For purpose of discussion, and in keeping with the discussion relative to fuel injectors, one example of a hydraulic amplification for a fuel injector can be found in the description of U.S. Pat. No. 5,697,554, which is titled “Metering Valve for Metering a Fluid,” and which issued on Dec. 16, 1997 (“the '554 patent”). The '554 patent discloses a fuel injector that includes a needle valve arranged to selectively open fluid passages through which fuel may be delivered into the power cylinder of an internal combustion engine. Operation of the needle valve is controlled by a piezoelectric actuator. As described in the '554 patent, a hydraulic displacement amplifier is disposed between the piezoelectric actuator and the needle valve for converting the actuating displacement of the actuator into an increased stroke of the needle valve.
Hydraulic amplification of the stroke of a piezoelectric actuator is a commonly used arrangement that can increase the effective stoke of a piezoelectric actuator. In general, hydraulic amplification in a fuel injector involves providing two hydraulic piston bores of different cross sectional areas that are fluidly connected to one another within the fuel injector. A larger plunger disposed in the larger of the two bores is typically mechanically connected to the piezoelectric actuator, and a smaller plunger disposed in the smaller of the two bores is connected to those components of the fuel injector that are actuated. During operation, an incompressible fluid is provided within the bores such that a relatively small displacement of the piezoelectric actuator causes motion of the larger plunger that compresses the fluid within the bores. The compressed fluid thus pushes on the smaller plunger to effect actuation of the fuel injector components. Because of the different cross section between the two bores, the displacement of the larger plunger is amplified at the smaller plunger.
Although hydraulic amplification can effectively increase the powered stroke of a piezoelectric actuator, the force provided by the actuator over the increased stroke is reduced. Additionally, hydraulic amplification arrangements may lack sufficient force at the initial portion of the stroke to open fuel injector valves in applications using relatively high injection pressures. Further, insofar as its essential components require precise machining of complicated features and subcomponents of the injector, the durability of the fuel injector may be compromised and the cost of the fuel injector may be increased.